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coachg
Joined: 10 Sep 2000
Posts: 3550
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 Posted: Fri Jun 22, 2018 12:40 pm Post subject: |
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| bluefish1 wrote: | coachg, Is yaw twist? The twist of the fin? Or are you saying the board, as in my number 3? The flatness of the board on the water is never always perfectly flat because the board is pitching from side to side.
Sorry I just brought pitch into the discussion. If someone brings up vector, I'm outta here. |
The flatness of the board is roll. Yaw is the board sliding slightly sideways-think of spin out to a lesser degree. There are two factors acting on the fin. The force of the wind & the direction of the board. When these line up-straight down wind-you have no lift. We can not plane going straight down wind because there is an even/equal flow over the fin. Start going across the wind & you have an uneven flow over the fin. The wind is pulling in one direction while the board is moving in another so you have an uneven flow over the fin.
Coachg |
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J64TWB
Joined: 24 Dec 2013
Posts: 1685
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| Posted: Fri Jun 22, 2018 12:48 pm Post subject: |
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| Okay, so yaw is micro spinout, but really just stern pressure in relation to the angle of attack. If this creates lift, wouldn't it be "sideways" lift (upwind lift), not "off the water" lift? |
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nw30
Joined: 21 Dec 2008
Posts: 6485
Location: The eye of the universe, Cen. Cal. coast
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| Posted: Fri Jun 22, 2018 1:45 pm Post subject: |
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Yaw can occur at the tail of the board (spinout sensation) or at the nose of the board (the nose not pointing perfectly in the direction you are moving) which occurs constantly while powered up. As said before, the sail doesn't pull just forward, but also downwind, so the pressure on the fin is not equal on both sides of it, there will always be a bit more pressure on the downwind side, making the fin asymmetrical in it's behavior, like an airplane wing,,,,,,, lift, but in this case horizontal lift, not so much vertical.
If you look at fins that have been used for a long time in the surf, you can notice that the bottom of the fin has been worn down by the sand, giving you a flat, fat bottom, where it used to be a sharp point. The fin still works. |
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U2U2U2
Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
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Ugly_Bird
Joined: 04 Nov 2008
Posts: 335
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| Posted: Fri Jun 22, 2018 7:43 pm Post subject: Re: opening a can of worms |
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| bluefish1 wrote: |
(I don’t get this, wouldn’t the lift be sideways not up?).
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Would be if a fin had asymmetric cross-section (like a plane wing). |
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U2U2U2
Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
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| Posted: Fri Jun 22, 2018 7:52 pm Post subject: Re: opening a can of worms |
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| Ugly_Bird wrote: | | bluefish1 wrote: |
(I don’t get this, wouldn’t the lift be sideways not up?).
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Would be if a fin had asymmetric cross-section (like a plane wing). |
Only about 10years old now ASYMETTRICAL fins on multi fined boards, and even longer on surf boards , Cross section .. what is that ?
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joethewindsufa
Joined: 10 Oct 2010
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U2U2U2
Joined: 06 Jul 2001
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Location: Shipsterns Bluff, Tasmania. Colorado
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| Posted: Fri Jun 22, 2018 9:37 pm Post subject: |
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I looked , scanned the link. I dont see a reference to fin, except drag is mentioned.
To the some: carry on
Part I – Forces and Moments
Kevin is back again on Figure 2. It shows the kinds of forces that nature confronts him with. His natural skills allow him to sense and react to these forces without really thinking about them. Most windsurfers are the same way but some, like myself and those to whom this paper is aimed, find that an analytical understanding is fun in itself and often helpful on the water.
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rigitrite
Joined: 19 Sep 2007
Posts: 520
Location: Kansas City
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| Posted: Fri Jun 22, 2018 11:03 pm Post subject: |
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The correct answer for your question is: "beyond the scope and depth of this forum."
There is definitely a PhD worthy paper involved if someone could develop an experimental set-up that could demonstrate whether a windsurfing fin creates lift or if viscous forces are the dominant factor, and that's what allows you to go upwind.
The real point is, it doesn't matter, and it will not affect how you sail, which ever one is true.
Just for grins, I've done a decent amount of research on this topic, and I'm 97.2% sure that viscous forces are all that matter, and that your fin does not, in fact create lift. Case in point: look at kiters: there's no way that the cafeteria tray they're riding is creating lift, but when they're going fast enough to allow viscous forces to dominate and digging in the rail, they can go upwind remarkably well using the lift from the kite.
This is a complex subject to say the least.
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U2U2U2
Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
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| Posted: Sat Jun 23, 2018 10:43 am Post subject: |
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| rigitrite wrote: | The correct answer for your question is: "beyond the scope and depth of this forum."
There is definitely a PhD worthy paper involved if someone could develop an experimental set-up that could demonstrate whether a windsurfing fin creates lift or if viscous forces are the dominant factor, and that's what allows you to go upwind.
The real point is, it doesn't matter, and it will not affect how you sail, which ever one is true.
Just for grins, I've done a decent amount of research on this topic, and I'm 97.2% sure that viscous forces are all that matter, and that your fin does not, in fact create lift. Case in point: look at kiters: there's no way that the cafeteria tray they're riding is creating lift, but when they're going fast enough to allow viscous forces to dominate and digging in the rail, they can go upwind remarkably well using the lift from the kite.
This is a complex subject to say the least. |
This would be #8 on the OP options. YES.
Nothing against the poster who mentioned Bernoulli, his name comes up frequently in fin/fluid discussions. Since his theory was presented in1738, the aircraft flew in 1903, and fins on windsurfers came about ..when,I see no relevance . Aircraft wings have some similarity to fins, like the monkey that has some DNA as humans do.
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